In MIMO technology, the capacity of wireless networks can be clearly improved by pro viding a plurality of antennas in order to carry out data transmission via diversity and multiplexing schemes. In a conventional communication system, normally a plurality of user devices (UEs) are distinguished from each other in a horizontal dimension by providing a plurality of antennas in a base station that are arranged in a one-dimensional array. For example, four emission antennas for simultaneously communicating with a plurality of UEs can be provided in a base station, and these four emission antennas are arranged in a 4×1 one-dimensional array.
However, for example, in a three-dimensional cell environment having a macro cell and a small cell, etc., UEs are positioned at different altitudes. For example, a plurality of UEs can be positioned on different floors of a building. In such a case, with an existing linear array that distinguishes a plurality of UEs only in a horizontal dimension, it is difficult to accurately distinguish a plurality of UEs that are positioned at different altitudes. In view of this, three-dimensional beamforming technology has been proposed in order to increase the throughput of the radio communication system by further improving the signal transmission quality between the base station and the mobile station, whereby it is possible to enhance the spatial diversity in the vertical dimension with such three-dimensional beamforming technology.
In three-dimensional beamforming technology, the base station uses a two-dimensional antenna array to communicate with a mobile station.
Compared to a one-dimensional antenna, in a two-dimensional antenna array, in addition to antennas being arranged in the horizontal dimension and horizontal beam control performed, antennas are also arranged in the vertical dimension (altitude direction) and beam control is performed, thereby achieving three-dimensional beamforming. By using three-dimensional beamforming, a higher beamforming gain can be obtained, and a good signal transmission quality can be obtained with mobile stations in different locations (especially in different high locations).
On the other hand, before the base station transmits downlink data to the mobile station, channel estimation is necessary. Specifically, the mobile station carries out channel estimation in accordance with a channel-state information—reference signal (CSI-RS) that is transmitted from the base station, the channel estimation result is fed back to the base station, and the base station transmits downlink data based on the channel estimation result. In a conventional communication system, the CSI-RS that is transmitted from the base station is not subject to beamforming. In other words, the CSI-RS that is transmitted in each direction is the same.